1. Technical Field
The present invention relates to a sheet feeding device that is used for an image forming apparatus such as a copying machine, a printer, a facsimile machine, a printing machine and a multi-functional peripheral, and in particular, to a sheet feeding device wherein a single sheet is separated to be fed out from a sheet bundle through air-blowing, and to an image forming apparatus equipped with the sheet feeding device.
2. Background of the Invention
In recent years, uses for a copying machine and a printer have been expanded, and sheets of various types including coated paper have come to be used. Some of these sheets of various types show strong sticking power between respective sheets when they are stacked as a sheet bundle, whereby, an occasion where it is difficult to surely prevent multiple-sheet feeding tends to be caused, in a sheet feeding device that feeds out sheets one sheet by one sheet with a friction roller.
Therefore, there has been proposed a method wherein an air-blowing outlet is provided on a flank of the stacked sheet bundle in the sheet conveyance direction, and air is blown against plural sheets on the upper part of the stacked sheet bundle from this air-blowing outlet, so that air may pass between the sheets and plural sheets may be separated from the sheet bundle.
The sheet separated in this manner is sent to an image forming section one sheet by one sheet by a sheet-feeding roller, and in the case of image forming apparatuses such as printing machines, some of them employ a conveying method for the uppermost sheet on the stacked sheet bundle by using an adsorption belt, while attracting the sheet through suction. What is popular generally in this method is one having the structure wherein an air-blowing outlet that blows air against a sheet leading edge from the downstream side in the sheet conveyance direction is provided separately, and air is blown against a sheet leading edge from this air-blowing outlet to separate a single sheet only.
However, a sheet having high smoothness like coated paper has a characteristic to stick to each other strongly under the environmental condition of high humidity, and therefore, if a flow of air blown from the side to gaps of sheets is weak, it is impossible to separate sticking sheets from the sheet bundle and to cause the sheets to be lifted.
To solve this problem, air-blowing to lift the sheets may be made to be strong. However, if the air-blowing is made to be strong, there is generated air pressure that lifts up not only the sheet to be stuck on an adsorption belt but also plural sheets from the lower portion to press them against the adsorption belt. When air that lifts up sheets enters the gaps between sheets and plural sheets are lifted up, even if trying to separate and scrape off the useless sheet by blowing air from the front under the aforesaid condition, it is not possible to separate properly because of a collision between the aforesaid air and air to lift up, resulting in a cause for multiple-sheet feeding.
To solve this problem, Unexamined Japanese Patent Application Publication No. 2008-239312, for example, discloses a sheet feeding device wherein, when a sheet is stuck to an adsorption belt, air coming from the side is stopped to blow only air from the front side against the sheet, and thereby, to scrape off sheets other than the sheet that is stuck to the adsorption belt.
If this technology is employed, a collision between air from the front side and air from the flank side is not caused, and sheets other than the sheet that is stuck to the adsorption belt are scraped off properly.
However, even when the sheet feeding device disclosed in Unexamined Japanese Patent Application Publication No. 2008-239312 is used, it is difficult to cope with all sheets having various sheet types and various sizes (including thickness). In particular, in the case of a sheet that is small in size and is thin in thickness, there is a problem that plural sheets fly up to cause multiple-sheet feeding when air from the flank side is stopped and air from the front side is blown. For this problem, the inventors of the present invention found out through experiments that flying up of sheets can be inhibited by blowing air in a small air flow quantity from the flank side without stopping the air flow from the flank side when blowing air from the front side.
However, in the case of a sheet that is large in size and is thin in thickness, there still is a problem that sheets including the second sheet and thereafter are blown up and multiple-sheet feeding and sheet jamming are caused even when an air flow quantity is made to be small, if blowing of air from the side is continued even after the uppermost sheet in a sheet bundle is stuck on the adsorption belt.
An objective of the invention is to solve the aforesaid problems and to provide a sheet feeding device that is free from the problem of multiple-sheet feeding even for sheets having various sizes (including thickness), and has stable sheet conveyance properties.